With an increase in capacity for information processing in recent years, various information recording technologies have been developed. Particularly, the surface recording density of a magnetic recording device such as a hard-disk drive using magnetic recording technology is continuing to increase at an annual rate of approximately 100%. Recently, an information recording capacity exceeding 250 GB per disk has been required in a magnetic disk having a diameter of 2.5 inches used in, for example, a hard-disk drive. In order to satisfy such a requirement, it is required to realize an information recording density exceeding 400 Gbits per square inch. In order to achieve a high recording density in a magnetic disk used in, for example, a hard-disk drive, it has been necessary to refine magnetic crystal grains constituting a magnetic recording layer for recording information signals and also to reduce the thickness of the magnetic recording layer. However, in the case of a conventionally commercialized magnetic disk of an in-plane magnetic recording system (also referred to as longitudinal magnetic recording system or horizontal magnetic recording system), as a result of the advance in refinement of magnetic crystal grains, there has arisen a thermal fluctuation phenomenon where the thermal stability of a recorded signal is degraded due to a superparamagnetism phenomenon to lose the recorded signal, and this has been a hindrance factor for an increase in recording density of the magnetic disk.
As one means for solving this hindrance factor, a magnetic recording medium for a perpendicular magnetic recording system is known. In the case of the perpendicular magnetic recording system, unlike the case of the in-plane magnetic recording system, the easy magnetization axis of a magnetic recording layer is adjusted so as to be oriented in a direction perpendicular to the surface of a substrate. As compared with the in-plane magnetic recording system, the perpendicular magnetic recording system can suppress the thermal fluctuation phenomenon and is therefore suitable for increasing the recording density.
However, the requirement for an increase in information recording capacity increasingly becomes high, and, accordingly, there is a demand for appearance of a recording system that can achieve an ultra-high recording density that exceeds the information recording density of the perpendicular magnetic recording system.
As one method therefor, thermally assisted magnetic recording is drawing attention. This thermally assisted magnetic recording is a sort of a recording system where a magnetic recording system and an optical recording system are combined. It magnetically records by giving heat energy to a recording medium through light irradiation and then stores by enlarging the coercive force of the recording medium by rapid cooling. Reproduction is performed magnetically at room temperature. According to this thermally assisted magnetic recording system, record-reproduction can be performed against a medium having high coercive force and excellent thermal fluctuation resistance, which does not allow the conventional magnetic recording system to record. Consequently, it is possible to refine magnetic crystal grains while maintaining the satisfactory heat stability, and, thereby, it is expected to achieve an ultra-high recording density that exceeds the information recording density of the conventional perpendicular magnetic recording system and improve the S/N ratio in high density recording.
Incidentally, the magnetic disk used in the current magnetic recording system has a protective layer and a lubricant layer on a magnetic recording layer formed on a substrate in order to ensure durability and reliability of the magnetic disk. In particular, the lubricant layer disposed on the outermost surface is required to have various characteristics such as long-term stability, chemical resistance, friction property, and heat-resistant property.
Also in the current magnetic disk, it is a challenge to provide a magnetic disk having a lubricant layer excellent in heat resistance to prevent fly-stiction failure or corrosion failure even at an extremely low flying height of 10 nm or less or a magnetic disk having a lubricant layer with good temperature characteristics and can exert a stable action over a wide temperature range. In particular, an improvement in heat resistance property of a lubricant used in the lubricant layer is an urgent issue.
For example, Japanese Unexamined Patent Application Publication No. 2000-311332 (Patent Literature 1) discloses a magnetic recording medium applied with a lubricant including a combination of a circular triphosphazene lubricant and perfluoropolyether lubricant to improve lubricant properties and CSS properties without decomposing lubricant even if using a low flying-height magnetic head. Japanese Unexamined Patent Application Publication No. 2003-132520 (Patent Literature 2) discloses a magnetic disk medium applied with a phosphazene lubricant having a phosphazene ring on at least one end of a perfluoropolyether main chain. Furthermore, Japanese Unexamined Patent Application Publication No. 2004-152460 (Patent Literature 3) discloses a magnetic disk having a highly adhesive lubricant layer that stably acts even at ultra-low flying height and can inhibit migration by using a lubricant including a combination of a perfluoropolyether compound having a phosphazene ring in an end group and a perfluoropolyether compound having a hydroxyl group in an end group.